The present invention is directed to a speed control structure and the control method for a work vehicle.
Such a work vehicle usually has, among other things, a speed change operating element for manipulation by an operator, means for detecting operating position of the speed change operating element, continuously variable speed change device that receives power from an engine of the work vehicle; speed change position detecting means for detecting a speed change operation position of the continuously variable speed change device; operating means for speed-shifting the continuously variable speed change device; control means for controlling the operating means.
There is a work vehicle with a mechanical servo control mechanism where the servo valve controls the servo cylinder that controls the swash plate of a variable displacement pump of a hydraulic type continuously variable speed change device to a position that corresponds to the operated position of a speed change pedal based on the pedal position detected by a valve control linkage and on the operated position of the swash plate of the pump detected by a control linkage. See, for example, JP11-91379. There is also a work vehicle with an electronic servo control where the control device controls flow of hydraulic fluid to a servo cylinder for the pump swash plate of a variable displacement pump such that the swash plate is operated to a position corresponding to the operated position of a speed change pedal based on the operated position detected by pedal sensor and the operated position of the swash plate detected by a swash plate sensor.
However, in the mechanical servo control mechanism or in the electronic servo control mechanism mentioned above, the swash plate controlled simply to move to a position corresponding to the operated position of the speed change pedal. Thus, the greater the deviation between the position corresponding to the operated position of the speed change pedal and the position of the swash plate, the greater the control delay tends to become, which results in a “feel” that may be improved and made it difficult to maintain high level of responsiveness.